The present invention relates to reactance circuits and, more particularly, to a circuit and method for producing a balanced, variable reactance across a pair of circuit terminals, the value of which is controllable.
There is a myriad of uses for a balance reactance circuit having a variable reactance that is controllable. One obvious use for such a circuit, to which the present invention is directed, is to provide a controllable reactance as a portion of an LC reactive tank circuit of an oscillator. By varying the reactance, the total reactance of the tank circuit can be varied to cause a change in the center operating frequency of the oscillator. The oscillator, in turn, may be utilized in the FM or video detector stages of a television receiver.
Contemporary technology is driving television receivers to be manufactured as single monolithic integrated circuitry. As such, the FM and video detector portion of the television receiver, to the extent possible, has to be integratable in order to produce a single monolithic chip circuit. The requirement also dictates that the detector oscillator of the video detection circuitry be as simple as possible; the less complex the circuitry the more room that is available on board the integrated chip for additional components of the television receiver. One method that has been proposed is the use of a LC oscillator operating at a center frequency that can be tuned within a predetermined range. The oscillator is tuned by electronically varying the reactance of the LC tank circuit of the oscillator. In particular, by varying the capacitance value of the LC tank, the oscillator's center frequency is changed. Therefore, a requirement for an integrated variable reactance circuit is established.
A prior art single ended variable capacitance circuit, U.S. Pat. No. 4,109,214, produces a single-ended variable capacitance at an output terminal thereof. However, because of the balanced nature of the oscillator that is required for performance in television receivers, the single-ended capacitive circuit of the prior art is not useful. Additionally, the prior art circuit requires that signal components be dissipated to ground which is undesirable in quality television receivers due to the fact that such grounded signals could be picked up and reproduced within the receiver to cause distortion of the received television signal.
Therefore, a need exists for a balanced variable reactance circuit the value of reactance being made electronically variable.